Field of the Invention
The invention relates to a brake system for a motor vehicle including electronically actuated brake actuators assigned to the wheels of the motor vehicle. A pedal unit includes a plurality of sensors generating sensor signals in response to an actuation of a brake pedal because of a desired driver demanded braking. At least one electronic control unit evaluates the sensor signals and generates desired signals for a brake actuation force to be applied by the brake actuators. At least one transmission channel connects the pedal unit, the brake actuators and the control unit to one another.
Recent demands made of brake systems in motor vehicles--such as anti-lock systems, vehicle stability systems, drive slip or traction control systems, so-called smart cruise controls, brake assistants, and so forth--together with the demand to reduce assembly and maintenance costs, which are quite considerable in the current hydraulic brake systems--have lead to the development of new, purely electrical brake systems (also known as brake by wire), as described for instance in German Patent DE 196 15 186 C1 which defines the generic type pertinent to the present case.
In such an electrical brake system, the driver is disconnected in terms of force from the brake; that is, the braking moment demand expressed by the driver is no longer transmitted directly in the form of force via a hydraulic system but rather only as a signal over an electrical line. With this signal, an electric brake actuator is controlled, which with the aid of an electric energy supply generates a force at a brake which via a friction element generates the desired braking moment. In the case of a disk brake, the force that generates the braking moment via friction is called the brake application force.
The brake actuator (or braking force actuator) is mounted directly on the applicable wheel of the motor vehicle, so that its brake application force is expediently not transmitted over great distances in the vehicle, while the devices for detecting the braking moment demand on the part of the driver and the brake controller are disposed at a central point, for instance in the vicinity of the brake pedal. Thus the brake system is distributed over the entire motor vehicle.
Ascertaining the driver demand for braking moment and the system controller are central elements of major significance for the safety of the brake system. The brake pedal with the sensors is the sole source for detecting the driver's braking wish. The driver's braking demand is calculated on the basis of that wish, in that the sensor values are compared with one another to determine their validity, and that with the aid of predetermined rules and characteristic curves, a desired braking value for controlling the brake actuators is calculated.
ABS control units in hydraulic brake systems meet the need for a central brake controller that is capable of varying the braking demands coming from the driver (ABS, TCS or traction control system functions, etc.). Such a variation must meet stringent safety requirements, since it can directly affect the use of the brakes. ABS control units are therefore as a rule equipped with two microcomputers which monitor one another (U.S. Pat. No. 4,745,542). Since these control units perform solely additional functions of the brake system that are not necessary for the basic braking by the driver, this control unit has a secure state; that is, if a relevant error occurs it can be put into this safe state, namely the "OFF" state. This requires merely that it be known that such an error be present; no information about the type of error is needed.